Composite semipermeable membrane

ABSTRACT

A composite semipermeable membrane is described, wherein said membrane comprises (1) a porous substrate, (2) an ultrathin film as a surface layer formed by polymerization by crosslinking of polyvinyl alcohol and an amino compound having at least two secondary amino groups using a polyfunctional crosslinking reagent capable of reacting with secondary amino groups and hydroxyl groups, and (3) a porous inner layer composed of water insoluble polyvinyl alcohol which is present between the porous substrate and the ultrathin film.

FIELD OF THE INVENTION

The present invention relates to a composite semipermeable membrane anda process for preparation thereof.

BACKGROUND OF THE INVENTION

Recently, various composite semipermeable membranes wherein asemipermeable ultrathin film is formed on a porous substrate have beenproposed as semipermeable membranes for reverse osmosis orultrafiltration. For example, a composite semipermeable membrane whereina dense ultrathin film composed of polyethyleneimine crosslinked by apolyfunctional crosslinking reagent such as isophthaloyl chloride isformed on a porous substrate has been disclosed in U.S. Pat. No.4,039,440. However, this composite semipermeable membrane isinsufficient in oxidation resistance, particularly, in chlorineresistance, and there are disadvantages, in the case of the solution tobe processed being sterilized, that the processing system is generallycomplicated and the processing costs become high because of therequirement for a dechlorination step prior to the membrane processingstep. On the other hand, a composite semipermeable membrane wherein anultrathin film composed of ethylenediamine modified polyepichlorohydrincrosslinked with the same crosslinking reagent as described above isformed on a porous substrate to improve chlorine resistance has beendisclosed in U.S. Pat. No. 4,005,012. However, this compositesemipermeable membrane has a problem in practical use because thechlorine resistance is only slightly improved as compared to U.S. Pat.No. 4,039,440, and the water permeability is small. Likewise, it hasbeen disclosed in U.S. Pat. No. 3,951,815 that a semipermeable ultrathinfilm can be formed by crosslinking polyvinyl alcohol with a crosslinkingreagent such as isophthaloyl chloride. However, this compositesemipermeable membrane has a fault in practical use that the chlorineresistance is also slightly improved and the water permeability issmall.

Further, the above described semipermeable membranes are not suitablefor use in carrying out desalting of aqueous solutions of salts having alow osmotic pressure, such as brackish water or industrial water, etc.,because useful performance of such membranes is only exhibited in thecase of operating under a high operation pressure of at least 40 kg/cm².That is, many of the known composite semipermeable membranes do not havea practically suitable water permeability, viz., at least as high as 0.5m³ /m² ·day under a low pressure operating condition of from 10 to 15kg/cm².

On the other hand, composite membranes prepared by coating a poroussubstrate with amino compound monomers, such as phenylenediamine,piperazine, etc., and thereafter crosslinking with an aldehyde or diacylchloride, have been disclosed in Office of Saline Water Research andDevelopment Progress Report, PB-253193 (1976) and PB-288387 (1978).However, these membranes are not satisfactory from the viewpoint ofwater permeability, even though certain kinds of piperazine-acidchloride composite membranes are comparatively excellent in chlorineresistance and show a comparatively excellent performance under lowpressure operating conditions. Also, it is difficult to completely coverfine pores on the substrate with the crosslinked polymer layer, becausethe amino compound monomer is applied to the porous substrate andthereafter it is polymerized by crosslinking. Consequently, membranedefects are easily caused, and composite semipermeable membranes havinggood reproducibility and performance are difficult to prepare.

SUMMARY OF THE INVENTION

The present invention has been accomplished in order to overcome theabove described various problems of heretofore known compositesemipermeable membranes, and it is an object of the present invention toprovide a composite semipermeable membrane having particularly excellentoxidation resistance and highly selective separation properties inaddition to large water permeability under low pressure operatingconditions, and a process for preparation thereof.

The composite semipermeable membrane according to this inventioncomprises (1) a porous substrate, (2) an ultrathin film as a surfacelayer formed by polymerization by crosslinking of polyvinyl alcohol andan amino compound having at least two secondary amino groups using apolyfunctional crosslinking reagent capable of reacting with secondaryamino groups and hydroxyl groups, and (3) a porous inner layer composedof water insoluble polyvinyl alcohol which is present between the poroussubstrate (1) and the ultrathin film (2).

This composite semipermeable membrane is prepared according to a processof the present invention which comprises coating or impregnating aporous substrate with an aqueous solution containing polyvinyl alcoholand an amino compound having at least 2 secondary amino groups in themolecule, thereafter contacting the porous substrate with apolyfunctional crosslinking reagent capable of reacting with secondaryamino groups and hydroxyl groups to polymerize the polyvinyl alcohol andthe amino compound by crosslinking, and then heating the product.

DETAILED DESCRIPTION OF THE INVENTION

The polyvinyl alcohol used in the present invention is a water solublepolymer preferably having a saponification value of from 87 to 99 mole%and a degree of polymerization of from 300 to 3000. Such polymers arecommercially available, e.g., Poval 224, a product of Kuraray Co. In thepresent invention, polyvinyl alcohol having a degree of polymerizationof more than 1000 is not suitably used, although the degree ofpolymerization is not critical.

The amino compound used in the present invention is an aliphatic,alicyclic, aromatic, or heteroaromatic compound having at least 2secondary amino groups in the molecule which has a water solubility ofat least 0.1% by weight at room temperature (e.g., about 15° to 35° C.)and a weight loss of 50% or more when heated to 130° C. for 30 minutes.The weight loss is measured by an evaporation test. The weight lossrepresentation is a factor showing removal of unreacted compounds fromthe porous inner layer. As such amino compounds, there are, for example,compounds represented by formula (I)

    R.sup.1 --NH--A--NH--R.sup.2                               (I)

wherein A represents an alkylene group having from 2 to 8 carbon atoms,a divalent alicyclic group, a divalent aromatic group, or a divalentheteroaromatic group, and R¹ and R² each represents an alkyl grouphaving from 1 to 3 carbon atoms;

compounds represented by formula (II) ##STR1## wherein R³ represents analkylene group having from 0 to 4 carbon atoms, R⁴ and R⁵ eachrepresents an alkyl group having from 1 to 3 carbon atoms, and i and jeach represents an integer of 0 to 4;

compounds represented by formula (III) ##STR2## wherein R⁶ and R⁷ eachrepresents an alkyl group having from 1 to 3 carbon atoms, k and l eachrepresents an integer of 0 to 4, and m represents an integer of 0 to 4;and

compounds represented by formula (IV) ##STR3## wherein R⁸ represents analkyl group having from 1 to 3 carbon atoms, n represents 1 or 2, and prepresents an integer of 0 to 4.

Preferred examples thereof include: N,N'-dimethylethylenediamine,N,N'-dimethylpropylenediamine, N,N'-dimethyl-m-phenylenediamine,N,N'-dimethyl-p-phenylenediamine, 2,6-dimethylaminopyridine, etc.,according to formula (I); ##STR4## according to formula (II); ##STR5##according to formula (III); and piperazine, 2-methylpiperazine,2,5-dimethylpiperazine, homopiperazine (hexahydrodiazepine), etc.,according to formula (IV). These amino compounds can be used alone or asa mixture of two or more thereof.

In the composite semipermeable membrane of the present invention,polyvinyl alcohol and an amino compound as described above arecrosslinked using a polyfunctional crosslinking agent to form asemipermeable dense ultrathin film on a porous substrate. Such anultrathin film is prepared by coating or impregnating the poroussubstrate with a solution of polyvinyl alcohol and the amino compound,and thereafter contacting the substrate with a polyfunctionalcrosslinking reagent to polymerize them by crosslinking.

As a solvent for producing a solution of the mixture of polyvinylalcohol and the amino compound (referred to as the "raw solution"hereinafter), water is suitably used. The raw solution is prepared suchthat the amino compound is contained therein in an amount of from 10 to500 parts by weight, and preferably from 20 to 300 parts by weight,based on 100 parts by weight of polyvinyl alcohol, and the total contentof the polyvinyl alcohol and the amino compound is from 0.05 to 10% byweight, and preferably from 0.1 to 5% by weight. This raw solution maycontain a surface active agent for reducing surface tension in the caseof coating or impregnating the porous substrate. Further, it may containa reagent for neutralizing by-products, for example, bases such assodium hydroxide, aqueous ammonia or sodium phosphate, etc., in the caseby-products such as hydrochloric acid are produced during crosslinkingwith the crosslinking reagent.

The porous substrate used in the present invention is suitably amembrane having an asymmetric structure wherein the surface pore size isgenerally from 50 to 5000 Å and the pure water flux after operation for1 hour under a pressure of 3.5 kg/cm² (referred to as the membraneconstant hereinafter) is at least 10⁻⁵ g/cm² -second-atmosphere, andpreferably from 10⁻⁴ to 0.1 g/cm² -second-atmosphere. For example,polysulfone, polyacrylonitrile, cellulose ester, polypropylene andpolyvinyl chloride can preferably be used. Particularly, polysulfone ispreferable. The porous membrane may be reinforced by backing with clothor a nonwoven web.

The amount of the raw solution containing the polyvinyl alcohol and theamino compound applied to the porous substrate should be sufficient toprovide from 0.05 to 5 g/cm², and preferably from 0.1 to 1 g/m²,calculated as solid content. If necessary, the amount of coating iscontrolled so as to be in the above described range by air drying ordraining after application of the raw solution to the substrate.

Various conventional coating methods can be employed in this invention.The typical method is that the raw solution is poured onto the poroussubstrate and then drained.

The polyfunctional crosslinking reagent used in the present invention isa compound having 2 or more functional groups capable of reacting withsecondary amino groups and hydroxyl groups, for example, one or morekinds of acid halide groups, halogen sulfonyl groups, N-haloformylgroups, haloformate groups and acid anhydride groups, etc., in themolecule. Preferred examples include isophthaloyl chloride,terephthaloyl chloride, trimesic acid chloride, trimellitic acidchloride, trimellitic acid chloride anhydride, 1,3-dichlorosulfonylbenzene, picolinic acid chloride, 5-chlorosulfonyl isophthaloylchloride, and piperazine-N,N-dicarboxylic acid dichloride. Trimesic acidchloride, trimellitic acid chloride, and isophthaloyl chloride areparticularly preferable.

The process for contacting the coated layer of the raw solutioncontaining the polyvinyl alcohol and the amino compound with the abovedescribed polyfunctional crosslinking reagent can comprise dissolvingthe crosslinking reagent in an organic solvent which is substantiallyimmiscible with the solvent for preparing the above described rawsolution, and contacting the coated layer with the resultingcrosslinking reagent solution, or the process can comprise contactingthe coated layer with a vapor of the crosslinking reagent.

In the process which comprises contacting the coated layer with thecrosslinking reagent solution, it is necessary that the solvent for thecrosslinking reagent does not dissolve and swell the porous substrate.Preferably, hydrocarbon solvents having a solubility parameter of 7.0 to9.0 are used. Preferably, aliphatic and alicyclic hydrocarbons havingfrom 5 to 8 carbon atoms are used, examples of which include pentane,hexane, heptane, octane, cyclopentane, cyclohexane, petroleum ether,etc. In addition, trichlorotrifluoroethane is a suitable solvent. Thecrosslinking reagent solution generally has a concentration of from 0.05to 10% by weight and, preferably 0.1 to 5% by weight. The temperatureand time of contacting with the raw solution coated layer can vary,according to the kind and concentration of the crosslinking reagent, theconcentration of the raw solution, and the kind of the amino compoundmonomer, etc., but contacting is generally carried out at from 10° C. to60° C., for example, for from 10 seconds to 10 minutes, and preferablyfrom 30 seconds to 5 minutes, at room temperature.

In the case of using the vapor of the crosslinking reagent, the vaporpressure of the crosslinking reagent in the vapor atmosphere isgenerally at least 0.1 mmHg, and preferably, at least 0.2 mmHg at atemperature of 100° C. or less, though it depends upon the kind of thecrosslinking reagent to be used and the contacting temperature. Thecontacting temperature is generally from 5° C. to 90° C., and preferablyfrom 20° C. to 70° C., and the contacting time is from 0.1 seconds to 30minutes, and preferably from 1 second to 5 minutes.

In order to carry out the crosslinking effectively to obtain a compositesemipermeable membrane having good performance, the contacting time andthe vapor pressure are preferably selected such that the value of "P logT" wherein P (mm Hg) is the vapor pressure of the crosslinking reagentat the contacting temperature and T is the contacting time (second), ispreferably at least 0.1 and, particularly at least 0.3. There is noupper limit required for the "P log T" values, but it is generally 1000or less.

Further, gases which do not participate in the crosslinking reaction,such as air, nitrogen, carbon dioxide, freon gas, or an inert gas, etc.,may be present in the case of contacting the raw solution coated layerwith the vapor of the crosslinking reagent.

In the present invention, since the raw solution coated layer on theporous substrate is brought into contact with the crosslinking reagentsolution which is substantially immiscible with the raw solution or withthe vapor of the crosslinking reagent, crosslinking of the polyvinylalcohol and the amino compound with the crosslinking reagent is carriedout by interfacial polymerization at the surface layer of the rawsolution coating layer, by which a semipermeable dense ultrathin film isformed as a surface layer. The thickness of the dense ultrathin film,which depends upon the concentrations of the polyvinyl alcohol and theamino compound in the raw solution and the contacting time with thecrosslinking reagent, is generally from 50 to 800 Å, and preferably from100 to 500 Å. If the ultrathin film is too thin, partial defects arecaused on the film surface. On the other hand, if it is too thick, thewater permeability deteriorates.

The porous substrate, coated or impregnated with the raw solution andthereafter brought into contact with the crosslinking reagent, asdescribed above, is then subjected to heat treatment. This heattreatment is carried out for the purpose of simultaneously completingthe crosslinking of the polyvinyl alcohol and the amino compound withthe crosslinking reagent in the ultrathin film, while insolubilizing (inwater) the unreacted polyvinyl alcohol which does not react in thecrosslinking reaction in the inner part of the raw solution coatinglayer, i.e., between the ultrathin film and the porous substrate, andvolatilizing the unreacted amino compound at the same time from theinner part of the raw solution coating layer. Accordingly, the heatingtemperature is chosen such that the polyvinyl alcohol is insolubilizedand the greater part (at least about 50 wt%) of the amino compound isvolatilized. Generally, it is from 80° to 180° C., and preferably from100° to 150° C. The heating time is from 1 to 60 minutes, and preferablyfrom 5 to 30 minutes. As a result of such heat treatment, the compositesemipermeable membrane formed has a very large water permeability ascompared with previously known composite semipermeable membranes becausethe unreacted amino compound is volatilized to form many fine pores inthe water-insoluble polyvinyl alcohol inner layer and the ultrathin filmis supported on this porous polyvinyl alcohol layer.

Further according to the present invention, since the amino compoundhaving secondary amino groups reacts with polyvinyl alcohol and/or thecrosslinking reagent to form crosslinkages, the membrane does notcontain active hydrogen, such as hydrogen of imino groups, and,consequently, it has high oxidation resistance, and particularly highchlorine resistance.

Moreover, the composite semipermeable membrane of the present inventionhas not only excellent water permeability under low operating pressure,but also excellent selective separation ability. Accordingly, it can beutilized for various uses, such as for desalting of brackish water, inthe food industry, for treatment of waste water, for separation of oilsand water, etc. From the viewpoint of preparation, the solution of thepolyvinyl alcohol and the amino compound can be uniformly applied to theporous substrate because it has a suitable high viscosity, and,consequently, fine pores on the substrate can be completely covered bysubjecting the polyvinyl alcohol and the amino compound to crosslinkingpolymerization to form a uniform ultrathin film. As a result, it ispossible to always prepare a composite semipermeable membrane havingdefinite membrane properties.

Below, the present invention is illustrated by reference to examples.However, the present invention is not limited thereto. In the examples,"rejection" means a value calculated by the following formula ##EQU1##

EXAMPLE 1

After a porous membrane substrate composed of polysulfone (P-3500,produced by Union Carbide Co.) was uniformly coated with an aqueoussolution containing 0.25% by weight of polyvinyl alcohol (Poval 224,produced by Kuraray Co.), 0.25% by weight ofN,N'-dimethylethylenediamine and 0.5% by weight of sodium hydroxide, itwas immersed in a 1 wt% solution of trimesic acid chloride in n-hexaneat 25° C. for 1 minute. After this, the substrate was taken out tovolatilize the n-hexane, and the membrane was subjected to heattreatment at 110° C. for 10 minutes in air.

The resulting composite semipermeable membrane was tested in a reverseosmosis test by supplying a 5000 ppm aqueous solution of magnesiumsulfate at a temperature of 25° C. under a pressure of 14 kg/cm² to theultrathin film side of the membranes. The water flux after 24 hours was1.01 m³ /m² ·day and the rejection was 90.0%. When continuous operationwas carried out subsequently for 150 hours, the performance of themembrane did not deteriorate.

EXAMPLES 2-6

Composite semipermeable membranes were obtained by the same procedure asin Example 1, except that the amino compounds shown in Table 1 belowwere used instead of N,N'-dimethylethylenediamine. The reverse osmosisperformance of these composite semipermeable membranes were measuredunder the same conditions as in Example 1, and the results are shown inTable 1.

                  TABLE 1                                                         ______________________________________                                                               Water Flux  Rejection                                  Example                                                                              Amino Compound  (m.sup.3 /m.sup.2 · day)                                                         (%)                                        ______________________________________                                        2      Homopiperazine* 1.11        97.9                                       3      Piperazine      1.20        99.0                                       4      2,5-Dimethylpiperazine                                                                        1.03        98.1                                       5      N,N'Dimethyl-m- 0.85        98.3                                              phenylenediamine                                                       6      1,3-Dipiperazylpropane                                                                        0.80        98.3                                       ______________________________________                                         ##STR6##                                                                 

EXAMPLES 7-9

Composite semipermeable membranes were obtained by the same procedure asin Example 1, except that a 1 wt% solution of the crosslinking reagentshown in Table 2 in n-hexane was used as a crosslinking reagent solutioninstead of the solution of trimesic acid chloride, and piperazine wasused as the amino compound instead of N,N'-dimethylethylenediamine. Thereverse osmosis performance of these composite semipermeable membraneswas measured under the same conditions as in Example 1, and the resultsare shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                Water Flux Rejection                                  Example Crosslinking Reagent                                                                          (m.sup.3 /m.sup.2 · day)                                                        (%)                                        ______________________________________                                        7       Isophthaloyl chloride                                                                         0.93       98.8                                       8       Isophthaloyl chloride/                                                                        0.99       97.8                                               benzene-m-disulfonic                                                          acid chloride (3/1)                                                   9       Isophthaloyl chloride/                                                                        1.23       98.9                                               trimesic acid chloride                                                        (3/1)                                                                 ______________________________________                                    

EXAMPLE 10

A composite semipermeable membrane was obtained by the same procedure asin Example 1 except that the substrate after being coated with the rawsolution was brought into contact with isophthaloyl chloride vapor underconditions such that the "P log T" value was 0.4. When the reverseosmosis performance of this composite semipermeable membrane wasmeasured under the same conditions as in Example 1, the water flux was0.87 m³ /m² ·day, and the rejection was 98.1%.

EXAMPLE 11

The composite semipermeable membrane obtained in Example 3 was immersedin a 100 ppm aqueous solution of chlorine having a pH of 11.3 at a roomtemperature for 3 days. The reverse osmosis performance before immersionand after immersion was evaluated. The composite semipermeable membranebefore immersion showed a water flux of 1.25 m³ /m² ·day and a rejectionof 98.9%, and after immersion showed a water flux of 1.25 m³ /m² ·dayand a rejection of 98.7%. Accordingly, the chlorine resistance wasexcellent.

EXAMPLE 12

The reverse osmosis performance of the composite semipermeable membraneobtained in Example 3 was measured by varying the operating pressure ofan aqueous solution of magnesium sulfate, an aqueous solution of sodiumchloride or an aqueous solution of sucrose, and the results are shown inTable 3, from which it is obvious that the composite semipermeablemembrane has a high selective separation property for inorganic salts.

                  TABLE 3                                                         ______________________________________                                                     Operating                                                        Aqueous Solution of                                                                        Pressure   Water Flux  Rejection                                 Inorganic Salt                                                                             (kg/cm.sup.2)                                                                            (m.sup.3 /m.sup.2 · day)                                                         (%)                                       ______________________________________                                        5000 ppm MgSO.sub.4                                                                        14         1.25        99.0                                      1000 ppm MgSO.sub.4                                                                        14         1.28        99.2                                      5000 ppm NaCl                                                                              14         1.96        26.0                                      5000 ppm MgSO.sub.4                                                                        42         3.86        99.5                                      5000 ppm NaCl                                                                              42         5.60        64.0                                      5000 ppm Sucrose                                                                           14         1.87        99.5                                      ______________________________________                                    

COMPARATIVE EXAMPLE 1

A composite semipermeable membrane was obtained using an aqueoussolution containing 1% by weight of polyvinyl alcohol and 1% by weightof sodium hydroxide as a raw solution, and which did not contain anamino compound, by processing with a solution of trimesic acid chlorideand heating by the same procedure as in Example 1. When the reverseosmosis performance of this composite semipermeable membrane wasmeasured under the same conditions as in Example 1, the water flux was0.86 m³ /m² ·day and the rejection was 49.3%.

COMPARATIVE EXAMPLE 2

A composite semipermeable membrane was obtained using an aqueoussolution containing 1% by weight of piperazine and 1% by weight ofsodium hydroxide as a raw solution, by processing with a solution oftrimesic acid chloride and heating by the same procedure as inExample 1. When the reverse osmosis performance of this compositesemipermeable membrane was measured under the same conditions as inExample 1, the water flux was 0.65 m³ /m² ·day and the rejection was98.1%.

While the invention has been described in detail and with reference tospecific embodiment thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A composite semipermeable membrane comprising (1)a porous substrate, (2) an ultrathin film as a surface layer formed bypolymerization by crosslinking of polyvinyl alcohol and an aminocompound having at least two secondary amino groups using apolyfunctional crosslinking reagent capable of reacting with secondaryamino groups and hydroxyl groups, and (3) a porous inner layer composedof water-insoluble polyvinyl alcohol which is present between the poroussubstrate and the ultrathin layer.
 2. A composite semipermeable membraneaccording to claim 1 wherein said amino compound is represented byformula (I)

    R.sup.1 --NH--A--NH--R.sup.2                               (I)

wherein A represents an alkylene group having from 2 to 8 carbon atoms,a divalent alicyclic group, a divalent aromatic group, or a divalentheteroaromatic group, and R¹ and R² each represents an alkyl grouphaving from 1 to 3 carbon atoms.
 3. A composite semipermeable membraneaccording to claim 1 wherein said amino compound is represented byformula (II) ##STR7## wherein R³ represents an alkylene group havingfrom 0 to 4 carbon atoms, R⁴ and R⁵ each represents an alkyl grouphaving from 1 to 3 carbon atoms, and i and j each represents an integerof 0 to
 4. 4. A composite semipermeable membrane according to claim 1wherein said amino compound is represented by formula (III) ##STR8##wherein R⁶ and R⁷ each represents an alkyl group having from 1 to 3carbon atoms, k and l each represents an integer of 0 to 4, and mrepresents an integer of 0 to
 4. 5. A composite semipermeable membraneaccording to claim 1 wherein said amino compound is represented byformula (IV) ##STR9## wherein R⁸ represents an alkyl group having from 1to 3 carbon atoms, n represents 1 or 2, and p represents an integer of 0to
 4. 6. A composite semipermeable membrane according to claim 1, 2, 3,4, or 5, wherein said polyfunctional crosslinking reagent has 2 or morefunctional groups selected from acid halide groups, N-haloformyl groups,haloformate groups, halogen sulfonyl groups and acid anhydride groups.7. A composite semipermeable membrane according to claim 1, 2, 3, 4, or5, wherein said porous substrate is composed of polysulfone,polyacrylonitrile, cellulose ester, polypropylene or polyvinyl chloride.8. A composite semipermeable membrane according to claim 1, 2, 3, 4, or5, wherein the porous substrate has a surface pore size of from 50 to500 Å and has a pure water flux of at least 10⁻⁵ g/cm²-second-atmosphere after operation for one hour under a pressure of 3.5kg/cm².
 9. A composite semipermeable membrane according to claim 1,wherein said polyvinyl alcohol is a water soluble polymer having asaponification value of from 87 to 99 mole % and a degree ofpolymerization of from 300 to
 3000. 10. A composite semipermeablemembrane according to claim 9, wherein said ultrathin film has athickness of from 50 to 800 Å.